The invention relates to a braking force generator for a hydraulic brake system comprising a force input element, which is connected to a brake pedal, a housing and a primary piston displaceable in the housing, wherein the primary piston together with the housing encloses a primary pressure chamber for generation of a hydraulic brake pressure, a pedal simulation device connected to the force input element, a pedal actuation detection device for detecting a pedal actuation and an actuating force booster stage for exerting an actuating force on the primary piston, wherein during normal operation the force input element is mechanically uncoupled from the primary piston and the actuating force booster stage in accordance with the detected pedal actuation exerts an actuating force on the primary piston and wherein in the event of failure of the actuating force booster stage the force input element is workingly connectable to the primary piston.
Such braking force generators are known from the background art and used in electrohydraulic brake systems, in which during normal operation a brake pedal to be actuated is completely mechanically uncoupled from the downstream brake system. This mechanical uncoupling of the brake pedal from the actual brake system has the advantage that the driver request according to a pedal actuation is detected by sensor equipment and in accordance with the detected pedal actuation the remaining brake system is electronically controlled. It is then possible to take into account further parameters characterizing the instantaneous state of the vehicle.
In such brake systems, however, in case electronic control or servo-power assistance components fail, it is a requirement to provide an emergency operating mode, in which even without the assistance of the actuating force booster stage, in particular without hydraulic or pneumatic servo-power assistance or without electronic sensor elements, a braking operation may be carried out. In such an emergency operating mode, a mechanical coupling between the brake pedal and the hydraulic brake circuit is then to be established relatively quickly in order by means of the direct mechanical coupling to be able to act upon the hydraulic brake system and initiate a braking operation.
From an embodiment (FIGS. 2 and 3) of the species-defining EP 1 003 658 B1, and corresponding U.S. Pat. No. 6,233,932, both of which are incorporated by reference herein, a braking force generator of the initially described type is known, which provides both the previously described normal operating mode and the emergency operating mode. During normal operation, a pedal actuation is detected by means of the pedal simulation device and in accordance with the detected pedal actuation an actuating force booster stage is activated, which then acts upon the actual hydraulic brake system. The uncoupling of brake pedal and primary piston of the hydraulic brake system is achieved by means of a non-self-locking threaded connection. For this purpose, a threaded sleeve rotates freely on a threaded bolt and hence enables a non-working linear movement between the force input element connected to the brake pedal and the primary piston. During emergency brake operation, i.e. in the event of failure of the actuating force booster stage, the threaded sleeve upon pedal actuation is displaced into a locking position, so that the threaded bolt coupled to the primary piston is no longer able to move freely relative to the threaded sleeve. Rather, the result is an interlocking of threaded sleeve and threaded bolt and ultimately a working connection between the force input element and the primary piston. The solution according to EP 1 003 658 B1 admittedly has the desired effect of an extensive mechanical uncoupling of force input element and primary piston during normal operation and moreover presents a rapid response characteristic in the case of the emergency operating mode, but its construction is relatively complicated and entails exact and therefore laborious manufacture, which adds to the cost thereof.
EP 1 003 658 B1 shows a further embodiment (FIG. 1), which, whilst being of a simple construction, has the drawback that in the emergency operating situation it responds only after a relatively long pedal stroke. A similar system is known from EP 1 070 006 B1, and corresponding U.S. Pat. No. 6,494,546, both of which are incorporated by reference herein. In this system also, the force input element in the emergency operating situation has to travel a relatively long distance before a braking effect is achieved. In other words, with this background art, in an emergency operating situation the brake pedal has to be pressed down relatively far before a braking effect is achieved.
From DE 36 00 729, and corresponding U.S. Pat. No. 4,667,476, both of which are incorporated by reference herein, a braking force generator is known, in which the force input element extends into a recess of the primary piston and during normal operation is held inside this recess at a slight distance from a corresponding lay-on surface of the primary piston. For this purpose, a spring arrangement is used, which is held under bias by means of the actuating force booster stage. In the event of a defect of the actuating force booster stage, the spring arrangement remains ineffective. If in such a situation the driver actuates the brake, he presses down the brake pedal initially without resistance until the force input element is applied against the primary piston. A further brake pedal actuation is then transmitted to the primary piston. However, for a moment the driver has the unpleasant sensation that the brake pedal may be depressed without resistance.